Recent years have witnessed increasing restrictions upon mercury lamps due to the problem of environmental pollution. As a result, as a light source to replace mercury lamps, solid-state light-source projectors have been widely developed and marketed in which an extremely strong excitation light such as LD (Laser Diode) light is irradiated upon a phosphor wheel in which a phosphor is combined with a disk-shaped substrate to excite the phosphor and emit fluorescent light for use as a light source (for example, Patent Document 1 (JP 5618130 B)).
Further, with advances in technological development and the development of projectors having higher luminance, the need for higher-output fluorescence has made necessary LD that feature still higher light output. Because LD light has a small light-emitting point and extremely high directivity, high-power light can be focused upon an exceptionally small light-emitting point. A phosphor that emits perfectly diffuse light has a small light-emitting surface because excitation occurs on a smaller area of irradiation of LD light. As a result, etendue, which is an index of the utilization efficiency of light that is found from the light-emitting area and the radiation solid angle, can be reduced and fluorescent light can be used more efficiently.
The ability to use light efficiently brings about many advantages such as improved image quality, reduced need for cooling, lower power consumption, and lower cost. In other words, benefits increase with a decrease of the area of the LD light that is irradiated upon a phosphor.